In the 3rd Generation Partnership Project (3GPP) that is a mobile communication system standardization project, attempts have been made to enhance Long Term Evolution (LTE) in order to meet rapidly increasing traffic demands.
As one of such attempts, it has been studied to use not only a frequency band (licensed band) for which a license is granted to an operator but also a frequency band (unlicensed band) for which a license is unnecessary in LTE communication.
In the unlicensed band, a listen-before-talk (LBT) procedure is required to avoid interference with other system (wireless LAN or the like) different from an LTE system or with an LTE system of other operator.
The LBT procedure is a procedure that confirms whether or not a frequency channel (carrier) of an unlicensed band is clear based on received power (interference power) and uses the channel only in the case of being confirmed as a clear channel (see Non-Patent Literature 1).